This invention relates to the generation of charged particles and more particularly to the use of the charged particles in electrostatic imaging.
Ions can be generated to form electrostatic images in a wide variety of ways. Common techniques include the use of air gap breakdown, corona discharges and spark discharges.
Air gap breakdown, i.e. discharges occuring in small gaps between a conductive surface and the surface of a dielectric material, are widely employed in the formulation of electrostatic images. Representative U.S. Pat. Nos. are G. R. Mott 3,208,076; R. F. Howell 3,438,053; E. W. Marshall 3,631,509; A. D. Brown, Jr. 3,662,396; R. T. Lamb 3,725,950; A. E. Bliss et al. 3,792,495; G. Krekow et al. 3,877,038; and R. F. Borelli 3,958,251.
In the case of air gap breakdown it is necessary that the gap spacing be maintained between about 0.2 and 0.8 mils in order to be able to operate with applied potentials at reasonable levels and maintain charge image integrity. Even then the latent charged image is not uniform, so the resultant electrostatically toned image lacks good definition and dot fill. The discharge in such devices depends on external circuit elements rather than inherent characteristics of the device. The disruptive nature of the air gap breakdown leads to a limited surface life in such a device.
An alternative to air gap breakdown is the corona discharge from a small diameter wire or a point source. Illustrative U.S. Pat. Nos. are P. Lee 3,358,289; Lee F. Frank 3,611,414; A. E. Jvirblis 3,623,123; P. J. McGill 3,715,762; H. Bresnik 3,765,027; and R. A. Fotland 3,961,564. Corona discharges are used almost exclusively in electrostatic copiers to charge photoconductors prior to exposure, as well as for discharging. These applications require large area blanket charging/discharging, as opposed to formation of discrete electrostatic images. Unfortunately, standard corona discharges provide limited currents. The maximum discharge current density heretofore obtained has been on the order of 10 microamperes per square centimeter. This can impose a severe printing speed limitation. In addition, coronas can create significant maintenance problems. Corona wires are small and fragile and easily broken. Because of their high operating potentials they collect dirt and dust and must be frequently cleaned or replaced.
Corona discharge devices which enjoy certain advantages over standard corona apparatus are disclosed in Sarid et al., U.S. Pat. Nos. 4,057,723; Wheeler et al. 4,068,284; and Sarid 4,110,614. These patents disclose various corona charging devices characterized by a conductive wire coated with a relatively thick dielectric material, in contact with or closely spaced from a further conductive member. A supply of positive and negative ions is generated in the air space surrounding the coated wire, and ions of a particular polarity are extracted by a direct current potential applied between the further conductive member and a counterelectrode. Such apparatus ovecomes many of the above-mentioned disadvantages of prior art corona charge and discharging devices but is unsuitable for electrostatic imaging. This limitation is inherent in the feature of large area charging, which does not permit formation of discrete, well-defined electrostatic images. This prior art corona device requires relatively high extraction potentials due to greater separation from the dielectric receptor, and provides exponential ion current outputs in contrast to the linear outputs of the present invention.
Another device particularly suitable for electrostatic imaging is disclosed in R. A. Fotland et al. U.S. Pat. No. 4,155,093. This patent discloses an ion generating device including a solid dielectric member, contacted on opposite sides by two planar electrodes. One of the electrodes contains one or more apertures or similar edge surfaces, located opposite the other electrode. A high voltage varying potential between the two electrodes generates a pool of positive and negative ions in the apertures, which ions may be extracted by means of a direct current potential between the apertured electrode and a counterelectrode. This apparatus is suitable for electrostatic imaging in that the apertures may be configured in a desired shape in order to create an electrostatic image of corresponding shape. A multiplexible imaging device may be created by patterning an array of opposing electrodes in a matrix crossover arrangement. This apparatus provides high quality, high speed electrostatic imaging, but achieves limited ion current outputs and is difficult to manufacture.
Accordingly, it is an object of the invention to facilitate the generation of ions, particularly at high current densities. A paramount object is to provide ion generation apparatus for use in electrostatic imaging.
Another object is to provide a reliable and stable source of ions. A related object is to provide an ion generating system which does not require critical periodic maintenance. Another related object is to simplify maintenance and eliminate the objectional characteristics of corona wires, including the fragility and tendency to collect dirt and dust.
A further object of the invention is to provide an easily controllable source of ions. A related object is to achieve a multiplexible source of ions using different sources to supply an alternating breakdown field and an ion extraction field.
Yet another object of the invention is to generate ion currents for use in producing electrostatic images in which charge image integrity is maintained. A related object is to achieve comparatively uniform charge images which can be toned with good definition. Further objects are increased electrostatic printing speed and suitable charge densities.
Still another object of the invention is to provide imaging apparatus with a desirably long service life. A related object is to avoid degradation of the imaging apparatus due to high voltage ion generation.